In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such process gas typically containing, among other components, carbon dioxide, CO2, oxygen gas, O2, and nitrogen oxides, NOx. With increasing environmental demands processes have been developed for removing carbon dioxide from the process gas and disposing of the removed carbon dioxide, in order to reduce the emission of carbon dioxide into the atmosphere. One such process, aimed at recovery of carbon dioxide of high purity from a carbon dioxide rich flue gas, involves compression of the flue gas, drying of the flue gas, liquefaction of carbon dioxide present in the flue gas and subsequent vaporization and compression of the carbon dioxide. A plant for performing such a process is often referred to as a gas processing unit, GPU.
An example of a gas processing unit is disclosed in EP 2 457 637 A1. The GPU of EP 2 457 637 A1 compresses a carbon dioxide rich flue gas coming from a gas cleaning system to an absolute pressure of at least 70 bar, making the carbon dioxide suitable for disposal, or suitable for being transported away for disposal. Such disposal could, for example, include pumping the carbon dioxide into a saline aquifer, pumping the carbon dioxide deep into the sea, or reusing the carbon dioxide in an industrial process. The disclosed GPU comprises, as its main units, a low pressure compression unit, an intermediate dehydration unit, an intermediate non-condensable gas removal unit, for example a CO2 liquefaction unit, and a high pressure compression unit.
A problem prone to appear in a compression unit is the presence of nitrogen oxides and water vapour in the gas to be treated, leading to the formation of nitric and nitrous acids in the compression unit. The acids formed may corrode the surfaces of equipment treating the flue gas in a GPU or may otherwise impact adversely the operation of equipment treating the flue gas in a GPU.
The energy demand of a gas processing unit has negative effect on the net efficiency degree of the power plant. Accordingly, there is a need for improvements as regards the energy utilization of a GPU.